Research The Propagation Rules Of Middle-High Frequency Electromagnetic Wave In Coal Medium

Geological forecasting work is the basic link in coal mine production. Geology andcoal seam structure factors influence the exploitation efficiency, and even threaten lives.Radio wave perspective as a coal mine geophysical method can effectively detect thegeological structures. But there are also issues in this method. Therefore, study thepropagation of electromagnetic wave in the coal-rock medium has important theoreticaland practical value. The main contents of the thesis are as follows:1) Based on Maxwell equations and from the "source", keep the linear loop antenna asa magnetic dipole, and established spherical coordinate system. Then computation theradiation field by make use of the Green theorem and the potential functionA. Using thedifferential current method to verify the equivalent magnetic dipole radiation field.Spherical wave equivalent to the plane wave, and established the basis for further study ofthe propagation rule of electromagnetic wave.2) When adopted a plane rectangular coordinate system and fixed point mode, remotereceiver point relative to emission point had a problem of incident angle. Thought aboutelectromagnetic wave from roadway space (air medium) incident into coal-rock media(lossy medium), calculated the attenuation constantpthat involved incident angle, andverified the equivalence of the classic attenuation constant andpwhen the incident angleequal to zero degree. The relationship between attenuation constantpand incident angle,conductivity, relative permittivity and the working frequency were presented. Also therelationship between incident angle and initial field intensity was discussed.3) Based on the total reflection law, when the refractive index of the coal is less thanthe surrounding rock’s the waveguide phenomenon was produced Then the expression ofeach component of the electromagnetic wave in the rectangular waveguide were given byusing longitudinal field method. Two kinds of waveguide modesH10andH20existed in thelong-wall mining working face. The relationship between attenuation constant ofconductor and working frequency were discussed, and conductor attenuationconstant csharp increasing when working frequency approached the cut-off frequency,with the working frequency increasing the cdecreases gradually and then increasing withthe working frequency continued rising and at a certain frequency chad an minimumvalue. When working frequency was constant, cincreased with the waveguide dimensionsdecreased. While attenuation constant dof medium depended on the characteristics of themedium which filling the waveguide and working wavelength, the amplitudes loss largerwith working wavelength approached cut-off wavelength. 4) The initial electric field intensity were calibrated and normalized which radiated bythe antenna. Then various models’ electromagnetic wave attenuation value wererespectively calculated using decay constant p, classic decay constant and interfaceeffect. Based on direct ray theory, the imaging of electromagnetic field intensityattenuation was completed by using Algebra Reconstruction Technology (ART). Andcompleted two different kinds of tomography imaging for different decay constants.Analysis the electromagnetic waves absorption of different decay constants, differenttomography imaging methods, and structures with different electrical resistivity, Theresults shown that,(1) For absolute tomography imaging, horizontal location was moreaccurate and decay value was more obvious by using decay constant p, but for relativetomography imaging, there was less redundant structures. Therefore, the constant p was moresuitable for relative tomography imaging.(2) For absolute tomography imaging, it waseasily to enlarge the range in vertical direction. While for relatively tomography imaging it waseasily to shrink the range in horizontal direction, and conduced losing real structure.(3) Theattenuation value of low resistivity model is greater than the high resistivity model, and thelocation was also better.(4) For3040mmodel, it got a good result, but as the dimension wasreduced to1520m, only the horizontal location accuracy can still meet the need.(5) When singlefault with vertical plane existed, the actual location and direction can be accurately confirmed. Whenmultiple faults existed. the effect was not obvious.Comprehensive analysis and comparison two attenuation constants and the imagingmethod, it can be seen that the attenuation constantpand absolute tomography method areeven better. Finally, three experiments and practical perspecting work were done by usingelectromagnetic perspective technology to detect the concealed structures which in thework face. Accuracy and usefulness were credible by using attenuation constantpinelectromagnetic tomography technique.